Backlight unit for backlit display

Abstract

To provide a backlight unit for a display with a backlight.SOLUTION: The present invention relates generally to a backlight unit of a backlit display, and specifically to a backlight unit for a backlit display with light emitting diode (LED) light sources. The backlight unit includes an array of light emitting diodes, at least two optical films positioned over the array of light emitting diodes, and a pair of brightness enhancement films positioned over the at least two optical films. Most of the optical films are light splitting optical films having a plurality of light splitting microstructures on at least one surface thereof.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This application claims the benefit of priority to U.S. Provisional Patent Application No. 62 / 898,693, filed September 11, 2019, U.S. Provisional Patent Application No. 62 / 929,309, filed November 1, 2019, and U.S. Provisional Patent Application No. 63 / 023,618, filed May 12, 2020, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates generally to backlight units for backlit displays, and more particularly to backlight units for backlit displays with light emitting diode (LED) light sources. [Background technology]

[0003] In pursuit of improved image quality, liquid crystal displays (LCDs) are increasingly using backlight unit architectures 100, illustrated diagrammatically in Figure 1, which include an array 110 of individual short-wavelength (blue) LEDs 112. Figures 2A and 2B illustrate a typical intensity distribution of light emitted from a single LED as a function of angle, as measured by a light distribution meter. As shown, the LED source approximates a Lambertian source, emitting a light distribution that is approximately symmetric about the nadir, with the highest light intensity at the nadir.

[0004] 1 , a series of films can be used to broaden or diffuse the light emitted from the blue LED 112 so that the backlight unit 100 can deliver more uniform light to an LCD panel (not shown) containing liquid crystals located above the backlight unit 100. As shown, the backlight unit 100 typically includes a diffuser film 120, which can be a volume diffuser or a circular diffuser; a color conversion layer 130, which uses either quantum dots or phosphor materials, for example, to convert a portion of the blue light emitted by the LED 110 to green and red light; a diffuser film 140, which can be a volume diffuser or a circular diffuser resulting from a randomly textured surface configured to broaden or diffuse the light exiting the color conversion layer 130; and two brightness enhancement films (BEFs) 150, 160, which are often two prismatic films rotated approximately 90 degrees relative to each other. Additional films in the backlight unit 100 can be present to improve the overall uniformity and brightness of the light being delivered to the LCD panel. In some backlight units, white LEDs may be used without a color conversion layer.

[0005] When LEDs 112 are arranged in an array, such as the array 110 illustrated in FIG. 3, it is desirable to obscure the individual LEDs 112 and present a bright, uniform light to the LCD panel. As noted above, one approach to achieving this goal is to include one or more diffusers, such as diffuser films 120, within the backlight unit 100 to diffuse, widen, or obscure the beam of light emitted by the LEDs 112. FIG. 4 diagrammatically illustrates such diffusion of light emitted by a single LED 112, with darker shades of gray representing brighter light than lighter shades of gray. Such diffusion can also reduce the average energy of the light.

[0006] Additionally, electronic devices, including LCDs, are becoming thinner and thinner. As a result, the backlight units of such displays are also becoming thinner and thinner, which presents another challenge in managing the light emitted by the LEDs 112 in an effective manner. For example, as shown diagrammatically in FIG. 5A , when a diffuser film 120 is placed over an array 110 of LEDs 112, the individual points of light emitted by the LEDs are diffused such that light with lower intensity from adjacent LEDs 112 begins to overlap, creating an area of ​​light with higher intensity. If the thickness of the diffuser film 120 is increased, which may not be desirable for thinner backlight units 100, the individual points of light may be spread even further, providing better uniformity of light, but there will still be brighter and darker areas, as shown diagrammatically in FIG. 5B .

[0007] It is desirable to have a backlight unit 100 for an LCD display having an array 110 of blue LEDs 112 and a low profile, yet deliver bright, uniform light to the LCD panel while still effectively hiding the individual LEDs 112. Summary of the Invention [Means for solving the problem]

[0008] According to one embodiment of the present invention, there is provided a backlight unit including an array of light-emitting diodes, at least two optical films positioned above the array of light-emitting diodes, and a pair of brightness enhancement films positioned above the at least two optical films, wherein a majority of the at least two optical films are light-dividing optical films having a plurality of light-dividing microstructures on at least one surface thereof.

[0009] In some embodiments, all of the at least two optical films have a plurality of light-dividing microstructures on at least one surface thereof.

[0010] In some embodiments, the backlight unit includes a color conversion layer positioned above the array of light-emitting diodes and below the pair of brightness enhancement films. In some embodiments, the color conversion layer is positioned above at least one light-splitting optical film. In some embodiments, the color conversion layer has at least one surface with a plurality of light-splitting microstructures.

[0011] In some embodiments, the backlight unit includes at least one additional light-splitting optical film positioned above the color conversion layer and below the pair of brightness enhancement films.

[0012] In some embodiments, the at least two optical films include a first light-dividing optical film having a plurality of first parallel linear prisms extending in a first direction on a first side thereof and a plurality of first elliptical lens structures extending in a second direction on a second side thereof. The second direction is substantially perpendicular to the first direction. The first side faces the array of light-emitting diodes. In some embodiments, the at least two optical films include a second light-dividing optical film positioned above the first light-dividing optical film. The second light-dividing optical film has a plurality of second parallel linear prisms extending substantially in the first direction on its first side thereof and a plurality of second elliptical lens structures extending in the second direction on its second side thereof. The first side of the second light-dividing optical film faces the second side of the first light-dividing optical film.

[0013] In some embodiments, the at least two optical films include a third light-dividing optical film positioned above the second light-dividing optical film. The third light-dividing optical film includes a plurality of third parallel linear prisms extending substantially in a second direction on a first side thereof. In some embodiments, the third light-dividing optical film further includes a plurality of microstructures on a second side thereof. In some embodiments, the second side of the third light-dividing optical film faces the second side of the second light-dividing optical film.

[0014] In some embodiments, at least one of the optical films is a first light-dividing optical film including a plurality of first parallel linear prisms extending in a first direction on a first side thereof and a plurality of second parallel linear prisms extending in the first direction on a second side thereof. In some embodiments, at least one of the optical films is a second light-dividing optical film including a plurality of first parallel linear prisms extending in the first direction on a first side thereof and a plurality of second parallel linear prisms extending in the first direction on a second side thereof. In some embodiments, at least one of the optical films is a second light-dividing optical film including a plurality of first parallel linear prisms extending in a second direction substantially perpendicular to the first direction on a first side thereof and a plurality of second parallel linear prisms extending in the second direction on a second side thereof.

[0015] In some embodiments, at least one of the optical films is a first light-dividing optical film including a plurality of first parallel linear prisms extending in a first direction on a first side thereof and a plurality of second parallel linear prisms extending in a second direction substantially perpendicular to the first direction on a second side thereof.

[0016] In some embodiments, two of the optical films are light-dividing optical films. Each light-dividing optical film includes a plurality of microstructures on a first side thereof and a plurality of parallel linear prisms extending in a first direction on a second side thereof. Each microstructure has a square pyramid shape.

[0017] In some embodiments, three of the optical films are light-dividing optical films. Each light-dividing optical film includes a plurality of microstructures on a first side thereof and a plurality of parallel linear prisms extending in a first direction on a second side thereof. Each microstructure has a square pyramid shape.

[0018] According to one aspect of the present invention, a backlight unit is provided that includes an array of light-emitting diodes and a lower stack of optical films positioned above the array of light-emitting diodes and configured to receive light emitted by the array of light-emitting diodes. The lower stack of optical films includes a first light-dividing optical film including a plurality of first light-dividing microstructures on a first side facing the array of light-emitting diodes, the plurality of first light-dividing microstructures being constructed and arranged to divide the light received from the array of light-emitting diodes. The lower stack of optical films includes a second light-dividing optical film positioned above the first light-dividing optical film. The second light-dividing optical film includes a plurality of second light-dividing microstructures on a first side facing the first light-dividing optical film, the plurality of second light-dividing microstructures being constructed and arranged to divide the light received from the first light-dividing optical film. The backlight unit includes a color conversion layer positioned above the lower stack of optical films and configured to receive light from the lower stack of optical films, an upper stack of optical films positioned above the color conversion layer and configured to receive light from the color conversion layer, and a pair of brightness enhancement films positioned above the upper stack of optical films and configured to receive light from the upper stack of optical films.

[0019] In an embodiment, the plurality of first light-splitting microstructures includes a plurality of first parallel linear prisms, and the plurality of second light-splitting microstructures includes a plurality of second parallel linear prisms oriented orthogonal to the plurality of first parallel linear prisms.

[0020] In some embodiments, the first light-dividing optical film also includes a plurality of first randomly roughened microstructures on its second side, and the second light-dividing optical film also includes a plurality of second randomly roughened microstructures on its second side.

[0021] In some embodiments, the lower stack of optical films also includes a third optical film positioned above the second light-dividing optical film. In some embodiments, the third optical film includes a plurality of microstructures facing the second light-dividing optical film. In some embodiments, each of the plurality of microstructures of the third optical film generally has the shape of a four-sided pyramid.

[0022] In some embodiments, the upper stack of optical films includes a third light-dividing optical film positioned above the color conversion layer, hi some embodiments, the upper stack of optical films also includes a fourth light-dividing optical film positioned above the third light-dividing optical film.

[0023] In some embodiments, the color conversion layer has at least one surface that includes a plurality of light-splitting microstructures.

[0024] These and other aspects, features, and characteristics of the present invention, as well as its method of operation and structure, function of associated elements and combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and appended claims, with reference to the accompanying drawings, all of which form a part of this specification. It is to be expressly understood, however, that the drawings are for the purpose of illustration and description only and are not intended as a definition of the limits of the invention. As used in this specification and claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. The present invention provides, for example, the following. (Item 1) A backlight unit, an array of light emitting diodes; at least two optical films positioned above the array of light emitting diodes; a pair of brightness enhancement films positioned above the at least two optical films; Equipped with A backlight unit, wherein a majority of the at least two optical films are light-dividing optical films having a plurality of light-dividing microstructures on at least one surface thereof. (Item 2) Item 2. The backlight unit of item 1, wherein all of the at least two optical films have the plurality of light-splitting microstructures on at least one surface thereof. (Item 3) Item 10. The backlight unit of item 1, further comprising a color conversion layer positioned above the array of light-emitting diodes and below the pair of brightness enhancement films. (Item 4) Item 4. The backlight unit of item 3, wherein the color conversion layer is positioned above at least one light-dividing optical film. (Item 5) Item 5. The backlight unit of item 4, wherein the color conversion layer has at least one surface with a plurality of light-splitting microstructures. (Item 6) Item 5. The backlight unit of item 4, further comprising at least one additional light-splitting optical film positioned above the color conversion layer and below the pair of brightness enhancement films. (Item 7) Item 1, a backlight unit, wherein the at least two optical films include a first light-dividing optical film having a plurality of first parallel linear prisms extending in a first direction on a first side thereof and a plurality of first elliptical lens structures extending in a second direction on a second side thereof, the second direction being approximately perpendicular to the first direction, and the first side facing the array of light-emitting diodes. (Item 8) 8. The backlight unit of claim 7, wherein the at least two optical films include a second light-dividing optical film positioned above the first light-dividing optical film, the second light-dividing optical film having a plurality of second parallel linear prisms extending substantially in the first direction on a first side thereof and a plurality of second elliptical lens structures extending in the second direction on a second side thereof, the first side of the second light-dividing optical film facing the second side of the first light-dividing optical film. (Item 9) Item 9. The backlight unit of item 8, wherein the at least two optical films include a third light-dividing optical film positioned above the second light-dividing optical film, the third light-dividing optical film having a plurality of third parallel linear prisms extending substantially in the second direction on a first side thereof. (Item 10) Item 10. The backlight unit of item 9, wherein the third light division further comprises a plurality of microstructures on its second side. (Item 11) Item 11. The backlight unit of item 10, wherein the second side of the third light-dividing optical film faces the second side of the second light-dividing optical film. (Item 12) Item 1, a backlight unit, wherein at least one of the optical films is a first light-dividing optical film having a plurality of first parallel linear prisms extending in a first direction on its first side and a plurality of second parallel linear prisms extending in the first direction on its second side. (Item 13) Item 13. The backlight unit of item 12, wherein at least one of the optical films is a second light-dividing optical film having a plurality of first parallel linear prisms extending in the first direction on its first side and a plurality of second parallel linear prisms extending in the first direction on its second side. (Item 14) Item 13. The backlight unit of item 12, wherein at least one of the optical films is a second light-dividing optical film having a plurality of first parallel linear prisms extending in a second direction on its first side that is approximately perpendicular to the first direction, and a plurality of second parallel linear prisms extending in the second direction on its second side. (Item 15) Item 1, a backlight unit, wherein at least one of the optical films is a first light-dividing optical film having a plurality of first parallel linear prisms extending in a first direction on its first side and a plurality of second parallel linear prisms extending in a second direction substantially perpendicular to the first direction on its second side. (Item 16) Item 1, a backlight unit, wherein two of the optical films are light-dividing optical films, each light-dividing optical film having a plurality of microstructures on its first side and a plurality of parallel linear prisms extending in a first direction on its second side, each microstructure having a square pyramid shape. (Item 17) Item 1, a backlight unit, wherein three of the optical films are light-dividing optical films, each light-dividing optical film having a plurality of microstructures on its first side and a plurality of parallel linear prisms extending in a first direction on its second side, each microstructure having a square pyramid shape. (Item 18) A backlight unit, an array of light emitting diodes; a lower stack of optical films positioned above the array of light emitting diodes and configured to receive light emitted by the array of light emitting diodes, the lower stack of optical films comprising: a first light-dividing optical film comprising a plurality of first light-dividing microstructures on a first side thereof facing the array of light-emitting diodes, the plurality of first light-dividing microstructures constructed and arranged to divide light received from the array of light-emitting diodes; a second light-dividing optical film positioned above the first light-dividing optical film and comprising a plurality of second light-dividing microstructures on a first side thereof facing the first light-dividing optical film, the plurality of second light-dividing microstructures constructed and arranged to divide light received from the first light-dividing optical film; a lower stack of optical films comprising: a color conversion layer positioned above the lower stack of optical films and configured to receive light from the lower stack of optical films; an upper stack of optical films positioned above the color conversion layer and configured to receive light from the color conversion layer; a pair of brightness enhancement films positioned above the upper stack of optical films and configured to receive light from the upper stack of optical films; A backlight unit comprising: (Item 19) Item 19. The backlight unit of item 18, wherein the plurality of first light-splitting microstructures comprises a plurality of first parallel linear prisms, and the plurality of second light-splitting microstructures comprises a plurality of second parallel linear prisms oriented perpendicular to the plurality of first parallel linear prisms. (Item 20) 20. The backlight unit of item 19, wherein the first light-dividing optical film further comprises a plurality of first randomly roughened microstructures on its second side, and the second light-dividing optical film further comprises a plurality of second randomly roughened microstructures on its second side. (Item 21) Item 19. The backlight unit of item 18, wherein the lower stack of optical films further comprises a third optical film positioned above the second light-dividing optical film. (Item 22) Item 22. The backlight unit of item 21, wherein the third optical film comprises a plurality of microstructures facing the second light-dividing optical film. (Item 23) Item 23. The backlight unit of item 22, wherein each of the plurality of microstructures of the third optical film generally has the shape of a four-sided pyramid. (Item 24) Item 19. The backlight unit of item 18, wherein the upper stack of optical films includes a third light-splitting optical film positioned above the color conversion layer. (Item 25) Item 25. The backlight unit of item 24, wherein the upper stack of optical films further comprises a fourth light-dividing optical film positioned above the third light-dividing optical film. (Item 26) Item 19. The backlight unit of item 18, wherein the color conversion layer has at least one surface with a plurality of light-splitting microstructures. [Brief explanation of the drawings]

[0025] The components in the following figures are illustrated to emphasize the general principles of the present disclosure and are not necessarily drawn to scale, although at least one of the figures may be drawn to scale. Reference characters designating corresponding components are repeated as necessary throughout the figures for consistency and clarity.

[0026] [Figure 1] FIG. 1 is a schematic diagram of a typical backlight unit containing an array of LEDs for an LCD display.

[0027] [Figure 2A] FIG. 2A is a three-dimensional plot of the distribution of light output from an LED as a function of angle, as measured by a photometer.

[0028] [Figure 2B] FIG. 2B is the measured light distribution of FIG. 2A represented in two dimensions.

[0029] [Figure 3] 3 is a schematic diagram of a top view of a portion of the array of LEDs of the backlight unit of FIG. 1. FIG.

[0030] [Figure 4] FIG. 4 is a schematic diagram of a top view of the distribution of light output from a single LED after the light has passed through a diffuser film.

[0031] [Figure 5A] FIG. 5A is a schematic diagram of a top view of the array of LEDs of FIG. 3 after the light emitted by the LEDs has passed through a diffuser film.

[0032] [Figure 5B] FIG. 5B is a schematic diagram of the array of LEDs of FIG. 3 after the light emitted by the LEDs has passed through a diffuser film having a thickness greater than the diffuser film used with respect to FIG. 5A.

[0033] [Figure 6] FIG. 6 is a schematic diagram of a backlight unit for an LCD display according to an embodiment of the present invention.

[0034] [Figure 7] FIG. 7 is a schematic diagram of a bottom stack of optical films of the backlight unit of FIG. 6, according to an embodiment of the present invention.

[0035] [Figure 8] FIG. 8 is a schematic diagram of two light-dividing optical films in the lower stack of optical films of FIG. 7, according to an embodiment of the present invention.

[0036] [Figure 9] FIG. 9 is a three-dimensional plot of the distribution of light output from an LED source having the light distribution of FIG. 2A after the light has passed through two light-splitting optical films of FIG. 8, as measured by a light distribution meter.

[0037] [Figure 10]FIG. 10 is a three-dimensional plot of the distribution of light output from an LED source having the light distribution of FIG. 2A after the light has passed through two light-splitting optical films of FIG. 8 that have higher refractive indices than the two light-splitting optical films having the light distribution of FIG. 9, as measured by a light distribution meter.

[0038] [Figure 11] FIG. 11 is a two-dimensional plot of the measured distribution of light output from an LED source having the measured light distribution of FIG. 10 and the light distribution of FIG. 2A after the light has passed through a circular diffuser.

[0039] [Figure 12A] FIG. 12A is a schematic diagram of a top view of the distribution of light output from a single LED after the light has passed through two light-splitting optical films of FIG.

[0040] [Figure 12B] 12B is a schematic diagram of a top view of a portion of the array of LEDs of FIG. 6 after the light emitted by the LEDs has passed through the light-splitting optical film of FIG.

[0041] [Figure 13] FIG. 13 is a three-dimensional plot of the distribution of light output from an LED source having the light distribution of FIG. 2A after the light has passed through two light-splitting optical films having the light distribution of FIG. 10 and a circular diffuser providing moderate diffusion, as measured by a light distribution meter.

[0042] [Figure 14] FIG. 14 is a three-dimensional plot of the distribution of light output from an LED source having the light distribution of FIG. 2A after the light has passed through two light-splitting optical films having the light distribution of FIG. 10 and a volume diffuser providing very high diffusion, as measured by a light distribution meter.

[0043] [Figure 15] FIG. 15 is a two-dimensional plot of the measured light distributions of FIGS.

[0044] [Figure 16] FIG. 16 is a schematic diagram of a third optical film of the lower stack of optical films of FIG. 7, in accordance with an embodiment of the present invention.

[0045] [Figure 17] FIG. 17 is a three-dimensional plot of the distribution of light output from an LED source having the light distribution of FIG. 2A after the light has passed through two light-splitting optical films having the light distribution of FIG. 10 and the third optical film of FIG. 16, as measured by a light distribution meter.

[0046] [Figure 18] FIG. 18 is an output plot from the modeling program showing the intensity of light from an LED light source as a function of position in two dimensions after the light has passed through two light-splitting optical films with high refractive indices and a volume diffuser.

[0047] [Figure 19] FIG. 19 is an output plot from the modeling program showing the intensity of light from an LED light source as a function of position in two dimensions after the light has passed through two light-splitting optical films having high refractive indices and the third optical film of FIG. 16 .

[0048] [Figure 20] FIG. 20 is an output plot from a modeling program showing the intensity of light from an LED light source as a function of position in two dimensions after the light has passed through another embodiment of two light-splitting optical films having high refractive indices and the third optical film of FIG.

[0049] [Figure 21A] FIG. 21A is a schematic diagram of a first side of a light-dividing optical film according to one embodiment of the present invention.

[0050] [Figure 21B]FIG. 21B is an enlarged photomicrograph of a portion of the second side of the light-dividing optical film of FIG. 21A.

[0051] [Figure 22A] FIG. 22A is a two-dimensional plot of the distribution of light output from an LED source having the light distribution of FIG. 2A after the light has passed through the single light-splitting optical film illustrated in FIGS. 21A and 21B, as measured by a light distribution meter.

[0052] [Figure 22B] FIG. 22B is a two-dimensional plot of the distribution of light output from an LED source having the light distribution of FIG. 2A after the light has passed through a single light-splitting optical film, according to one embodiment of the present invention, as measured by a light distribution meter.

[0053] [Figure 22C] FIG. 22C is a two-dimensional plot of the distribution of light output from an LED source having the light distribution of FIG. 2A after the light has passed through a single light-splitting optical film, according to one embodiment of the present invention, as measured by a light distribution meter.

[0054] [Figure 23] FIG. 23 is a schematic diagram of a light-dividing optical film according to one embodiment of the present invention.

[0055] [Figure 24] FIG. 24 is a schematic diagram of a light-dividing optical film according to one embodiment of the present invention.

[0056] [Figure 25] FIG. 25 is a schematic diagram of a light-dividing optical film according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0057] Detailed Description 6 schematically illustrates a portion of a backlight unit 600, according to an embodiment of the present invention. As shown, the backlight unit 600 includes an array 610 of LEDs 612, which may be the same blue-emitting LEDs 112 as described above, a lower stack of optical films 620, a color conversion layer 630 above the lower stack of optical films 620, an upper stack of optical films 640, which may include one or more diffuser films above the color conversion layer 630, a first brightness enhancement film (“BEF”) 650 above the upper stack of optical films 640, and a second brightness enhancement film (“BEF”) 660 above the first BEF 650. The first BEF 650 and the second BEF 660 have substantially identical structures, as is known in the art, but may be rotated 90° relative to each other. The color conversion layer 630 may include, for example, phosphors or quantum dots, as known in the art, and may be configured to change the wavelength of some of the light being emitted from the LED 612, such as from blue wavelengths to red and green wavelengths.

[0058] 7 is a more detailed schematic diagram of the lower stack of optical films 620 of FIG. 6. As shown, the lower stack of optical films 620, which are optical films positioned between the LED 612 and the color conversion layer 630, includes a first light-dividing optical film 622, a second light-dividing optical film 624, and an optional third optical film 626. The third optical film 626 may be, for example, a volume diffuser film or another light-dividing optical film, as described in more detail below. Additional optical films may be used in the lower stack of optical films 620. The illustrated embodiment is not intended to be limiting in any way.

[0059] As defined herein, an "optical film" is a polymeric film. As defined herein, a "light-splitting optical film" is a polymeric film that includes a plurality of light-splitting microlenses or microstructures on at least one surface. As defined herein, a "light-splitting microstructure" is a microstructure that, when a collimated beam is directed on-axis relative to the microstructure, splits the collimated beam into two or more beams with regions of lower relative intensity on-axis.

[0060] For example, the light-splitting microstructure may be in the form of a prism and split an incident beam into two beams, with the angle between the two beams depending on the prism angle and the refractive index of the prism material. In some embodiments, a prism with a 90-degree angle and a refractive index of 1.5 may split an incident on-axis beam into two beams at approximately ±25 degrees. In some embodiments, the light-splitting microstructure may be in the form of a three-sided pyramid and split an incident on-axis beam into three beams. In some embodiments, the light-splitting microstructure may be in the form of a four-sided pyramid and split an incident on-axis beam into four beams. In some embodiments, the light-splitting microstructure may be in the form of a cone and split an incident on-axis beam into conical rings.

[0061] Such light-splitting microstructures may be fabricated using many techniques known in the art. For example, in some embodiments, the shape of the light-splitting microstructure may be cast onto a substrate using a suitable master mold and a thermosetting or ultraviolet (UV) light-curable polymer, or the shape may be imprinted into a thermoplastic substrate through compression molding or other molding, or may be created simultaneously with the substrate using extrusion-embossing or injection molding. The microstructure may be generated by replicating a master. For example, optical films may be made by copying a master containing the desired features, as described in commonly assigned U.S. Pat. No. 7,190,387 B2 to Rinehart et al., entitled "Systems And Methods for Fabricating Optical Microstructures Using a Cylindrical Platform and a Rastered Radiation Beam," U.S. Pat. No. 7,867,695 B2 to Freese et al., entitled "Methods for Mastering Microstructures Through a Substrate Using Negative Photoresist," and / or U.S. Pat. No. 7,192,692 B2 to Wood et al., entitled "Methods for Fabricating Microstructures by Imaging a Radiation Sensitive Layer Sandwiched Between Outer Layers," all of which are incorporated by reference in their entirety as if fully set forth herein. The master itself may be processed using the laser scanning techniques described in these patents, and may also be copied to provide the microstructures using the copying techniques described in these patents.

[0062] In some embodiments, laser holography, as known in the art, may be used to create holographic patterns that create desired microstructures in photosensitive materials. In some embodiments, projection or contact photolithography, such as that used in semiconductors, displays, circuit boards, and other common technologies known in the art, may be used to expose microstructures into photosensitive materials. In some embodiments, laser ablation, either using a mask or using a focused and modulated laser beam, may be used to create microstructures, including indicia, in materials. In some embodiments, micromachining, also known as diamond machining, as known in the art, may be used to create desired microstructures from solid materials. In some embodiments, additive manufacturing, also known as 3D printing, as known in the art, may be used to create desired microstructures in solid materials.

[0063] 8 schematically illustrates an embodiment of a first light-dividing optical film 810 and a second light-dividing optical film 820 that can be used as the first light-dividing optical film 622 and the second light-dividing optical film 624 of FIG. 7. The first light-dividing optical film 810 is configured to receive individual beams of light emitted by the array of LEDs 610 and split each beam of light into two beams of light. The second light-dividing optical film 820 is configured to receive the beams of light from the first light-dividing optical film 810 and split each beam of light into two beams of light, thereby causing the individual beams of light received by the first light-dividing optical film 810 to be split into four beams of light upon exiting the second light-dividing optical film 820. 8 , the first light-dividing optical film 810 includes a plurality of light-dividing microstructures 812 in the form of parallel linear prisms extending across one side of the first light-dividing optical film 810 facing downward (and toward the array of LEDs, not shown). The first light-dividing optical film 810 also includes a plurality of randomly roughened microstructures 814 on the side opposite the parallel linear prisms 812. Similarly, the second light-dividing optical film 820 includes a plurality of light-dividing microstructures 822 in the form of parallel linear prisms extending across one side of the second light-dividing optical film 820 facing downward and toward the first light-dividing optical film 810, and a plurality of randomly roughened microstructures 824 on the side opposite the parallel linear prisms 822.

[0064] The first light-dividing optical film 810 and the second light-dividing optical film 820 are oriented relative to each other such that the plurality of light-dividing microstructures 812 of the first light-dividing optical film 810 are oriented 90° relative to the plurality of light-dividing microstructures 822 of the second light-dividing optical film 820, which allows the original beam of light from each LED (see FIG. 2A ) to be split into four beams of light, as measured by a light distribution meter and illustrated in FIG. 9. By increasing the refractive index of the plurality of light-dividing microstructures 812, 822, the original beam of light from each LED can be split into four beams of light and further widened, as measured by a light distribution meter and illustrated in FIG. 10.

[0065] Figure 11 is a two-dimensional plot of the measured light distribution of Figure 10 (represented by 1100) and the measured distribution of light output from an LED source having the light distribution of Figure 2A after the light has passed through a circular diffuser (represented by 1110). As shown, the pair of light-dividing optical films 810, 820 divides the light received from the LED, spreading it more widely (i.e., away from the nadir at 0°) than a circular diffuser and also suppressing on-axis (i.e., nadir) light compared to a circular diffuser. On-axis light is suppressed by reflecting it back toward the LED, which helps to conceal the LED from being seen above the pair of light-dividing optical films 810, 820.

[0066] Figure 12A schematically illustrates the effect of two light-splitting optical films 810, 820 on light emitted by a single LED 612, in contrast to the effect of a circular diffuser on light emitted by a single LED 112 shown schematically in Figure 4. Figure 12B schematically illustrates the effect of two light-splitting optical films 810, 820 on light emitted by an array 610 of LEDs 612, in contrast to the effect of a circular diffuser on an array 110 of LEDs 112 shown schematically in Figures 5A and 5B. As depicted, the light output by the two light-splitting optical films 810, 820 is generally brighter and more uniform than the light output by a circular diffuser.

[0067] To investigate further improvements in the uniformity of the light output from the pair of light-splitting optical films 810, 820, a circular diffuser providing moderate diffusion was placed over the pair of light-splitting optical films 810, 820 having a higher refractive index (output illustrated in FIG. 10), and the light passing through the stack of three films was measured using a light distribution meter. The results are illustrated in FIG. 13 and show that the Gaussian diffusion after the light is split four ways by the pair of light-splitting optical films 810, 820 appears to suppress much of the desirable spread of the light created by the pair of light-splitting optical films 810, 820.

[0068] A volume diffuser providing very high diffusion was placed over the pair of light-splitting optical films 810, 820 having the higher refractive index, and the light passing through the stack of three films was measured using a light distribution meter. The results are illustrated in Figure 14 and show that the increased diffusion after the light is split four ways by the pair of light-splitting optical films 810, 820 appears to further inhibit the desired spread of light created by the pair of light-splitting optical films 810, 820.

[0069] Figure 15 is a two-dimensional plot of the measured light distributions of Figures 10, 13, and 14. More specifically, Figure 15 illustrates a comparison of two-dimensional light intensity distributions for light emitting from a pair of light-dividing optical films 810, 820 having a higher refractive index (represented by 1100), from a circular diffuser film providing moderate diffusion (represented by 1500), and from a volume diffuser providing very high diffusion (represented by 1510), showing that increasing diffusion reduces the desired spread provided by the pair of light-dividing optical films 810, 820.

[0070] Similar effects seen with circular diffusers and volume diffusers were also found with color conversion layer 630. Specifically, phosphor films may also suppress some of the desired spread of light created by two or more light dividing optical films 810, 820. Therefore, in addition to the lower stack of films 620, it may also be desirable to use a pair of light dividing optical films 810, 820 (and in some embodiments, a single light dividing optical film) above color conversion layer 630 in the upper stack of optical films 640, and / or add light dividing microstructures to one or both surfaces of color conversion layer 630.

[0071] FIG. 16 schematically illustrates an optical film 1600 that may be used as the third optical film 626 in the lower stack of optical films 620, according to an embodiment of the present invention. As illustrated, the optical film 1600 includes a plurality of microstructures 1610 in the form of a four-sided pyramid on one side thereof. The optical film 1600 was placed on top of a pair of light-dividing optical films 810, 820 with a higher refractive index, with the plurality of microstructures 1610 facing the pair of light-dividing optical films 810, 820. Light passing through the stack of three films 810, 820, 1600 was measured using a light distribution meter. The results, illustrated in FIG. 17, show that after the light is split into four by the pair of light-dividing optical films 810, 820, the optical film 1600 with the plurality of microstructures 1610 in the form of a four-sided pyramid increases the uniformity of the light spreading provided by the light-dividing optical films 810, 820 in both directions, which is desirable. In some embodiments, an optical film 1600 having a plurality of microstructures 1610 in the form of square pyramids may be used in place of the pair of light-splitting optical films 810, 820.

[0072] To further investigate the effect of stacks of optical films according to embodiments of the present invention, LightTools lighting design software by Synopsis, Inc. was used to model the effect of various stacks of three optical films 622, 624, 626 in the bottom stack of optical films 620 on the point spread function (“PSF”), which is the intensity of light as a function of position (in x and y coordinates) above the third optical film 626. FIG. 18 illustrates modeling results using a pair of light-splitting optical films 810, 820 and a third film in the form of a volume diffuser that provides very high diffusivity. FIG. 18 illustrates a relatively narrow point spread function (PSF), similar to that measured using the light distribution meter of FIG. 14.

[0073] Figure 19 illustrates the results of using a pair of light-splitting optical films 810, 820 and a third optical film 1600 having a plurality of microstructures 1610. Similar to what was measured using the light distribution meter of Figure 17, Figure 19 illustrates the maintenance of high angular spread with the plurality of microstructures 1610 (square pyramids) compared to the results of the volume diffuser of Figure 18.

[0074] 20 illustrates modeling results when using two crossed prism films with a high refractive index similar to the pair of light-dividing optical films 810, 820 described above, but without the plurality of randomly roughened structures 814, 824, and a third optical film 1600 with a plurality of microstructures 1610. As defined herein, "high refractive index" means a refractive index greater than 1.65, such as 1.7. As shown, the crossed film with prisms without the randomly roughened microstructures on one side thereof provides less uniformity than the pair of light-dividing optical films 810, 820 with the randomly roughened microstructures 814, 824, resulting in four distinct spots ( FIG. 20 ) compared to the larger single spot ( FIG. 19 ). [Example]

[0075] To test the effects of different combinations of films in backlight unit 600, a series of combinations of optical films were used for lower stack of optical films 620 and upper stack of optical films 640, with identical color conversion layers 630 (phosphor films) between lower stack of optical films 620 and upper stack of optical films 640. The films used for lower stack of optical films 620 and upper stack of optical films 640 were a pair of light-splitting optical films having multiple microstructures and a pair of diffuser films in the form of a volume diffuser, respectively. Four different combinations were used, as summarized in Table 1 below. [Table 1]

[0076] Each of Examples 1-4 was mounted on a light board containing an array of mini-LEDs spaced 1.6 mm apart. When a pair of crossed (i.e., oriented 90° relative to each other) light-splitting optical films was used in the upper stack of optical films, the crossed light-splitting optical film pair as a unit was rotated approximately 20° clockwise relative to the array of mini-LEDs. The total thickness of each stack, the relative average energy emerging from the stack, and the range / average energy of each stack were measured. The results are summarized in Table II below. [Table 2]

[0077] A higher relative average energy indicates brighter light exiting the backlight unit 600, which is desirable, and a lower range / average energy indicates more uniform light exiting the backlight unit 600, which is also desirable. Test results show that backlight units 600 including two crossed light-splitting optical films in the upper stack 640 of optical films (Examples 3 and 4) had significantly higher average energy and significantly lower range / average energy exiting the stack compared to backlight units 600 including two volume diffusers in the upper stack (Examples 1 and 2). Example 4, which had two crossed light-splitting optical films in both the lower stack 620 of optical films and the upper stack 640 of optical films, had the thinnest thickness, highest average energy, and lowest range / average energy, which is desirable.

[0078] Additional samples were fabricated to investigate other combinations of films for the lower stack 620 of films in the backlight unit 600, as well as different spacings for the array 610 of light-emitting diodes 612. For Example 5, a stack of three light-dividing optical films was used for the lower stack 620 of optical films. A light-dividing optical film 2100 having the structure illustrated in FIGS. 21A and 21B and a total thickness of about 0.11 mm was used as the first light-dividing optical film 622. As shown, the light-dividing optical film 2100 includes a plurality of parallel linear prisms 2112 (see FIG. 21A ) extending in a first direction FD on a first side 2110 of the light-dividing optical film 2100, and a plurality of elliptical lens microstructures 2122 having an extent of 1° by 60° and extending in a second direction SD substantially perpendicular to the first direction FD, provided on a second side 2120 of the light-dividing optical film 2100. The prisms 2112 were made from a material with a refractive index of about 1.7. For the second light-dividing optical film 624 for Example 5, the same light-dividing optical film 2100 was used, but with a total thickness of about 0.2 mm. The multiple parallel linear prisms 2112 per film were aligned approximately parallel to each other in the first direction FD and oriented to face the array 610 of LEDs 612, compared to the orientation illustrated in FIG. 8. FIG. 22A illustrates a two-dimensional plot of the distribution of light output from the LEDs 612 having a Lambertian distribution after the light passes through the light-dividing optical film 2100 of FIGS. 21A and 21B with the multiple parallel linear prisms facing the LEDs 612. Lighter colors indicate higher light intensity.

[0079] Example 5 also included a third light-dividing optical film as third optical film 626, which included a plurality of randomized conical microstructures on a first side facing second light-dividing optical film 624 and a plurality of parallel linear prisms on a second side of third light-dividing optical film 626 opposite the first side. The prisms were made from a material having a refractive index of 1.7, and third light-dividing optical film 626 had a thickness of 0.2 mm.

[0080] For Example 6, four light-dividing optical films were used for the lower stack of optical films 620. The first light-dividing optical film 622 for this embodiment had multiple linear prisms on the bottom side facing the array 610 of LEDs 612 and multiple circular light-dividing microstructures on the top side of the first light-dividing optical film 622. The first light-dividing optical film 622 for this embodiment had a thickness of 0.17 mm, and the prisms were made from a material with a refractive index of approximately 1.7. FIG. 22B illustrates a two-dimensional plot of the distribution of light output from the LEDs 612, which has a Lambertian distribution, after the light passes through the first light-dividing optical film of this embodiment with multiple parallel linear prisms facing the LEDs 612. Lighter colors indicate higher light intensity.

[0081] The second light-dividing optical film 624 for this embodiment had a plurality of parallel linear prisms on its bottom side facing the array 610 of LEDs 612 and a plurality of randomized conical microstructures on its top side. The second light-dividing optical film 624 for this embodiment had a thickness of 0.12 mm, and the prisms were made from a material with a refractive index of approximately 1.7. Figure 22C illustrates a two-dimensional plot of the distribution of light output from the LEDs 612 having a Lambertian distribution after the light passed through the second light-dividing optical film of this embodiment with a plurality of parallel linear prisms facing the LEDs 612. Lighter colors indicate higher light intensity.

[0082] The second light-dividing optical film 624 was oriented relative to the first light-dividing optical film 622 such that the multiple parallel linear prisms of the second light-dividing optical film 624 were approximately perpendicular to the multiple parallel linear prisms of the first light-dividing optical film 622, similar to that illustrated in FIG. 8.

[0083] The third light-dividing optical film 626 for this embodiment had a plurality of circular light-dividing microstructures on its bottom side facing the second light-dividing optical film 624 and a plurality of parallel linear prisms on its top side. The film had a thickness of 0.11 mm, and the prisms were made of a material with a refractive index of approximately 1.7. The plurality of parallel linear prisms of the third light-dividing optical film were oriented parallel to the plurality of parallel linear prisms of the second light-dividing optical film 624. The fourth light-dividing optical film was identical to the third light-dividing optical film 626, but the plurality of parallel linear prisms were oriented approximately perpendicular to the plurality of parallel linear prisms of the third light-dividing optical film 626.

[0084] Also included in Examples 5 and 6 was a phosphor film used for the color conversion layer 630 and positioned above the third light-dividing optical film 626, having a thickness of 0.12 mm, and a pair of cross brightness enhancement films 650, 660, each having a thickness of 0.1 mm, positioned above the color conversion layer 630. No upper stack of optical films 640 was used between the color conversion layer 630 and the pair of brightness enhancement films 650, 660. A summary of the light-dividing optical films used for Examples 5 and 6 is summarized in Table III below. [Table 3]

[0085] Examples 5 and 6 were each mounted on a light board containing an array of mini-LEDs spaced 2.4 mm apart. The total thickness of each stack (including the color conversion layer and brightness enhancement film), the relative average energy emerging from the stack, and the range / average energy of each stack were measured. The results are summarized in Table IV below. [Table 4]

[0086] The test results for Examples 5 and 6 show that, even though the two lower stacks of optical films had the same thickness, the backlight unit 600 (Example 5) including three light-splitting optical films in the lower stack 620 of optical films had a greater average energy (higher brightness) and a lower range / average energy (higher uniformity) emitting from the stack compared to the backlight unit 600 (Example 6) including four light-splitting optical films in the lower stack 620 of optical films.

[0087] Test results indicate that it may be advantageous to use two or more light-splitting optical films in the lower stack of optical films 620 having elliptical lens structures on their top surfaces and parallel linear prisms on their bottom surfaces, with the parallel linear prisms for the two films oriented in substantially the same direction, i.e., within 30 degrees, or preferably within 15 degrees. While the elliptical lens structures described above had an extent of 1° x 60°, other shapes may also be used. For example, elliptical lens structures having an extent of 1° x 40° or 1° x 90° may be used according to embodiments of the present invention.

[0088] 23 schematically illustrates an embodiment of a light-dividing optical film 2300 that may be used as one or more of the light-dividing optical films 622, 624 in the bottom stack of optical films 620 illustrated in FIGS. 6 and 7. As shown, the light-dividing optical film 2300 includes a plurality of parallel linear prisms 2312 extending in a first direction FD on a first side 2310 of the light-dividing optical film 2300 and a plurality of parallel linear prisms 2322 similarly extending in the first direction FD on a second side 2320 of the light-dividing optical film 2300. In an embodiment, when two of the light-dividing optical films 2300 are used as the first and second light-dividing optical films 622, 624 of the bottom stack of optical films 620, all of the parallel linear prisms 2312, 2322 of both films 2300 may be aligned in substantially the same direction, e.g., the first direction FD.

[0089] In some embodiments, when two of the light-dividing optical films 2300 are used as the first and second light-dividing optical films 622, 624 in the bottom stack of optical films 620, one of the two light-dividing optical films 2300 may be oriented such that the plurality of linear prisms 2312, 2322 of one film are aligned substantially orthogonal to the plurality of linear prisms 2312, 2322 of the other film. For example, one film 2300 may have its plurality of linear prisms 2312, 2322 aligned in a first direction FD, while the other film may have its plurality of linear prisms 2312, 2322 aligned in a second direction SD that is substantially orthogonal to the first direction FD.

[0090] In one embodiment, when two of the light-dividing optical films 2300 are used as the first and second light-dividing optical films 622, 624 of the lower stack of optical films 620, one of the two light-dividing optical films 2300 may be oriented so that its plurality of linear prisms 2312, 2322 is aligned in a first direction FD, while the other film has its plurality of linear prisms 2312, 2322 aligned in any direction relative to the first direction FD, for example, in a direction between the first direction FD and the second direction SD.

[0091] 24 schematically illustrates an embodiment of a light-dividing optical film 2400 that may be used as one or more of the light-dividing optical films 622, 624 in the bottom stack of optical films 620 illustrated in FIGS. 6 and 7. As shown, the light-dividing optical film 2400 includes a plurality of parallel linear prisms 2412 extending in a first direction FD on a first side 2410 of the light-dividing optical film 2400 and a plurality of parallel linear prisms 2422 extending in a second direction SD, generally perpendicular to the first direction FD, on a second side 2420 of the light-dividing optical film 2400. In an embodiment, when two of the light-dividing optical films 2400 are used as the first and second light-dividing optical films 622, 624 of the bottom stack of optical films 620, all of the parallel linear prisms 2412 on the first side 2410 of the film 2400 may be aligned in substantially the same direction, e.g., the first direction FD.

[0092] In one embodiment, when two of the light-dividing optical films 2400 are used as the first and second light-dividing optical films 622, 624 of the lower stack of optical films 620, one of the two light-dividing optical films 2400 may be oriented so that the plurality of linear prisms 2412 on its first side 2410 is aligned approximately perpendicular to the plurality of linear prisms 2412 on the first side 2410 of the other film 2400, such that one film has its plurality of linear prisms 2412 aligned in a first direction FD, while the other film has its plurality of linear prisms 2412 aligned in a second direction SD that is approximately perpendicular to the first direction FD.

[0093] In one embodiment, when two of the light-dividing optical films 2400 are used as the first and second light-dividing optical films 622, 624 of the lower stack of optical films 620, one of the two light-dividing optical films 2400 may be oriented so that its plurality of linear prisms 2412 on the first side 2410 is aligned in the first direction FD, while the other film is aligned so that its plurality of linear prisms 2412 on the first side 2410 is aligned in any direction relative to the first direction FD, for example, in a direction between the first direction FD and the second direction SD.

[0094] 25 schematically illustrates an embodiment of a light-dividing optical film 2500 that may be used as one or more of the light-dividing optical films 622, 624 in the bottom stack 620 of optical films illustrated in FIGS. 6 and 7. As shown, the light-dividing optical film 2500 includes a plurality of four-sided pyramids 1610, as described above with reference to FIG. 16, on a first side 2510 of the light-dividing optical film 2500, and a plurality of parallel linear prisms 2522 extending in a first direction FD on a second side 2520 of the light-dividing optical film 2500. In an embodiment, when two of the light-dividing optical films 2500 are used as the first and second light-dividing optical films 622, 624 of the bottom stack 620 of optical films, all of the parallel linear prisms 2522 on the second side 2520 of the film 2500 may be aligned in substantially the same direction, e.g., the first direction FD.

[0095] In one embodiment, when two of the light-dividing optical films 2500 are used as the first and second light-dividing optical films 622, 624 of the lower stack of optical films 620, one of the two light-dividing optical films 2500 may be oriented so that the plurality of linear prisms 2520 on its second side 2520 is aligned approximately perpendicular to the plurality of linear prisms 2522 on the second side 2520 of the other film 2500, such that one film has its plurality of linear prisms 2522 aligned in a first direction FD, while the other film has its plurality of linear prisms 2522 aligned in a second direction SD that is approximately perpendicular to the first direction FD.

[0096] In one embodiment, the third optical film 626 of the lower stack of optical films 620 of Figures 6 and 7 is also the light-splitting optical film 2500 of Figure 25, and the multiple linear prisms 2522 may be aligned in the first direction FD or the second direction SD.

[0097] The embodiments described herein represent some possible implementations and examples and are not intended to necessarily limit the present disclosure to any specific embodiment. Instead, as will be understood by those skilled in the art, various modifications can be made to these embodiments, even if not explicitly described, and different combinations of the various embodiments described herein can be used as part of the present invention. For example, the light-splitting optical film and the diffuser optical film may include different microstructures and different combinations of microstructures than those depicted in the drawings, such as the microstructures disclosed in International Patent Application Publication No. WO 2019 / 152382 (the entire contents of which are incorporated herein).

[0098] Additionally, the top stack of optical films 640 may include the same or a different combination of films as the bottom stack of optical films 620. In some embodiments, most of the films in the backlight unit 600 located below the brightness enhancement films 650, 660 may have microstructures configured to split an incident beam of light into two or more beams of light. In some embodiments, all or nearly all of the optical films in the backlight unit 600 may have microstructures on at least one surface thereof configured to split an incident beam of light into two or more beams of light. The resulting brightness and uniformity of the light exiting the bottom stack of optical films 620 may be adjusted by using different combinations of prisms and microstructures on the various optical films in the bottom stack of optical films 620.

[0099] The illustrated and described embodiments above are not intended to be limiting in any way, and any such modifications of the embodiments described herein are intended to be within the spirit and scope of this disclosure and protected by the following claims.

Claims

1. A backlight unit, comprising: an array of light emitting diodes; at least two optical films positioned above the array of light emitting diodes; at least one brightness enhancement film positioned above the at least two optical films; Equipped with one or more of the at least two optical films is a light-dividing optical film having a plurality of light-dividing microstructures on at least one surface thereof, and at least one of the plurality of light-dividing microstructures is formed in a pyramidal shape; The backlight unit, wherein the plurality of light-splitting microstructures are configured such that when a collimated beam is directed on-axis at the plurality of light-splitting microstructures, the collimated beam is split into two or more beams.

2. A backlight unit as described in claim 1, wherein at least one of the plurality of light-dividing microstructures is formed in the shape of a quadrilateral pyramid.

3. A backlight unit as described in claim 2, wherein at least one of the plurality of light-splitting microstructures is formed in the shape of a triangular pyramid.

4. A backlight unit as described in claim 2, wherein at least one of the plurality of light-dividing microstructures is formed on a surface of the light-dividing optical film, and the surface of the light-dividing optical film faces downward.

5. A backlight unit as described in claim 2, wherein all of the at least two optical films have the plurality of light-splitting microstructures on at least one surface thereof.

6. The backlight unit of claim 2, further comprising a color conversion layer, the color conversion layer being positioned above the array of light-emitting diodes and below the at least one brightness enhancement film.

7. A backlight unit as described in claim 6, wherein the color conversion layer is positioned above at least one light-splitting optical film.

8. A backlight unit as described in Claim 7, wherein the color conversion layer has at least one surface having a plurality of light-splitting microstructures.

9. The backlight unit of claim 7, further comprising at least one additional light-dividing optical film, the at least one additional light-dividing optical film being positioned above the color conversion layer and at least below the brightness enhancement film.

10. A backlight unit as described in claim 2, wherein the at least two optical films include a first light-dividing optical film, a second light-dividing optical film positioned above the first light-dividing optical film, and a third light-dividing optical film positioned above the second light-dividing optical film, and the third light-dividing optical film has a plurality of parallel linear prisms on its first surface.

11. A backlight unit as described in claim 10, wherein the third light-dividing optical film further comprises a plurality of microstructures on its second surface.

12. A backlight unit as described in Claim 11, wherein the second surface of the third light-dividing optical film faces the second light-dividing optical film.

13. A backlight unit as described in claim 10, wherein the third light-dividing optical film has a plurality of microstructures on a first surface facing the second light-dividing optical film and a plurality of linear prisms on a second surface of the third light-dividing optical film opposite the first surface.

14. A backlight unit as described in claim 2, wherein at least one of the at least two optical films has a plurality of film microstructures facing away from the light-emitting diode.

15. A backlight unit as described in claim 2, wherein at least one of the at least two optical films has a plurality of microstructures on a first surface, the first surface facing a plurality of microstructures on another of the at least two optical films.

16. A backlight unit as described in claim 2, wherein at least one of the at least two optical films comprises a plurality of first parallel linear prisms extending in a first direction on its first surface and a plurality of second parallel linear prisms extending in the first direction on its second surface.

17. A backlight unit as described in claim 16, wherein at least one of the at least two optical films comprises a plurality of first parallel linear prisms extending in a second direction substantially perpendicular to the first direction on its first surface, and a plurality of second parallel linear prisms on its second surface.

18. A backlight unit as described in claim 2, wherein the at least two optical films comprise three light-dividing optical films, each light-dividing optical film comprising a plurality of microstructures on its first surface and a plurality of parallel linear prisms extending in a first direction on its second surface, each microstructure having a square pyramid shape.

19. A backlight unit as described in claim 2, wherein the at least one brightness enhancement film positioned above the at least two optical films includes a pair of brightness enhancement films.

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